One of the most costly aspects of IC technology is bonding the IC chip to the rest of the circuit package. A proposed solution to this problem comprises automated bonding of metal beam leads to the IC chip, thus avoiding manual wire bonding. Such a technique uses a roll of a laminate that carries to a bonding station a plurality of frames each of which has many generally planar microscopic, spider-shaped metal fingers known as beam leads. At the bonding station the inner portions of the beam leads are aligned and then simultaneously connected to the bonding sites on the IC chip. The outer portion of the beam leads is then available for bonding to remaining portions of the circuit package.
Such beam leads can be manufactured by a photoresist process. U.S. Pat. No. 4,247,623 discloses such a process and a blank or laminate for use therein having a structure comprising a flexible strip of electrically conductive metal, a layer of positive-working resist adhered to one surface of the metal strip, and a layer of negative-working resist adhered to the opposite surface of the metal strip. The process includes the steps of imagewise exposing and developing the positive-working resist and the negative-working resist. The developed positive resist forms a protective coating on the beam lead portions of the metallic layer. The exposed metallic layer is subsequently etched to form the beam leads. The developed negative resist forms a window-bearing support spacer holding the beam leads in the desired orientation. The beam leads are maintained in their proper orientation prior to and during bonding by the spacer formed from the negative-working resist.
Commonly owned U.S. patent application Ser. No. 77,715, now U.S. Pat. No. 4,792,517 entitled LAMINATE FOR THE FORMATION OF BEAM LEADS FOR IC CHIP BONDING, filed July 24, 1987 discloses a laminate featuring an improved negative-working resist exhibiting good flexibility and adhesion to the metal and excellent performance during high temperature processing whereby it serves effectively as a support for the beam leads.
The above-described laminates contain positive-working resists comprising light-sensitive cresol formaldehyde resins condensed with quinone diazides in a binder such as a poly(acrylic acid) or a copolymer of ethyl acrylate and methacrylic acid. Such positive-working resists provide, under most conditions, excellent laminates. However, their performance under all conditions has not been entirely satisfactory. For example, such resists tend to be brittle, which can lead to pick-off problems during coating and cracking due to development. Furthermore, such resists have a tendency to exhibit poor adhesion to the metal which can lead to delamination during the multiple bending and flexing that is characteristic of the processing of such laminates.
Thus, the problem of this invention has been to provide a laminate for the formation of beam leads for IC chip bonding featuring a positive-working resist having the advantages, including good resolving power and etch resistance, of the positive-workng resists described above, yet featuring improved mechanical properties.